As is well known, optical signal processing is important to the development of optical fiber telecommunications systems, since it allows realizing complex functions that are currently obtained with electronic systems. For this purpose, devices that are capable of performing different transfer functions between input and output optical signals are required.
Non-linear transfer functions are particularly interesting and are more difficult to achieve. These transfer functions to the functions in the electronics field, allow devices to be made to treat optical signals, like threshold comparators, amplitude limiters, pulse shapers, oscillators, memories. Combinational and sequential logic functions, such as NAND, NOR, flip-flop, etc., are equally interesting for such devices.
It is also very useful to realize these functions in devices that act as optical waveguides for transmitted signals, since it is technologically much easier to send to, or to get from, these devices the radiation coming from another guiding system, like optical fibers. Moreover, optical waveguides are particularly well suited to be arranged on a common plane with lasers, optical amplifiers, electro-optical modulators, etc., to be integrated into the same substrate.
Non-linear transfer functions can be accomplished by a device called SEED (Self Electrooptic Effect Device), composed of an electro-optical modulator negatively fed back through an electronic circuit. Should this feedback be positive, such device shows a characteristic of the light absorption as a function of the inverted voltage applied, at constant wavelength, with an interval where absorption decreases upon voltage increase. This condition causes the photocurrent crossing the device to affect the device capacity to absorb light, that in turn affects the photocurrent. This results in an optical bistability that allows obtaining non-linear transfer functions.
Such a device is described in the article entitled "The quantum well self-electrooptic effect device: optoelectronic bistability and oscillation, and self-linearized modulation" by D. A. B. Miller et al., IEEE Journal of quantum electronics, Vol. 21, September 85, pages 1462-1476. The electro-optical modulator used is a PIN diode, where the active part is a heterostructure containing a sequence of uncoupled quantum wells. The system does not have waveguide geometry, nor can it be modified to operate in a waveguide geometry due to its low transmittivity to optical signals. To solve this, it would be necessary to provide extremely short guides (20 .mu.m), and this is a very difficult technological problem.
OBJECT OF THE INVENTION
It is an object of the invention to provide an optical processing device for light radiation which allows realizing the above-mentioned functions in an optical waveguide, enabling the fabrication of integrated optical circuits suitable to perform particularly complex logic and processing functions and facilitating optical fiber coupling.